(a) Field of the Invention
The present invention relates to a half tone liquid crystal display circuit and, more particularly, to a technology which is effective when applied to a color liquid crystal display circuit having a TFT active matrix structure for multi-color displays by a digital method.
(b) Description of the Prior Art
The color liquid crystal display circuit having the active matrix structure mounting TFTs (i.e., Thin Film Transistors) thereon is disclosed on pp. 211 of "Nikkei Electronics" published on Sept. 10, 1984 by Nikkei McGraw-Hill, for example.
The TFT liquid crystal display circuit is used as a small-sized low-power-consumption display circuit mainly for a monitor in a microcomputer system but is highly desired to have half-tone multi-color display as the display circuit in an office automation system.
For the half tone display using the liquid crystal display panel of the TFT active matrix structure, it is necessary to use a linear region in the brightness-voltage characteristics of the liquid crystal. However, the brightness-voltage characteristics in the liquid crystal highly fluctuate in dependence upon the vertical observation angle, as shown in FIG. 31. The angle of view or observation angle range, in which the color tone is maintained over a half tone, is found as small as about 9 degrees if it is determined from the transmissivity of each scale set at an observation angle of 0 degrees for the display panel. As shown in FIG. 31, each scale is varied in the direction to have the transmissivity reduced as a whole, i.e., toward the black level. As a result, the multi-color display to express delicate color tones for 512 colors, for example, is not practically possible.
In case, therefore, the observation angle is varied, as above, it is possible to vary the driving voltages corresponding to the individual scales. In this case, according to the simplest concept, it is possible to make the driving voltages adjustable to correspond to the individual scale displays. According to this adjustment, however, as many as eight portions have to be adjusted each time the observation angle is varied for the display of eight scales. The number of these combinations is so great as to prevent practical applications. For these reasons, the color liquid crystal display circuit of the prior art prepares the eight colors by combining the monotones of red, green and blue colors without using the linear portion of the aforementioned brightness-voltage characteristics. In this monotone case, the driving voltages can be generated with margins sufficient for preventing the fluctuations of the aforementioned brightness-voltage characteristics using the observation angle.
We have discovered that the brightness (transmissivity)-voltage characteristics are approximately varied with a constant reference voltage against the aforementioned vertical observation angle in the liquid crystal. By making use of this reference voltage, moreover, we have developed a display driving voltage generator which makes adjustments for the variations of the observation angle for the half tone displays by using the region in which the transmissivity of the liquid crystal linearly varies.
An object of the present invention is to provide a half tone liquid crystal display circuit which adjusts the half tone display simply and, accurately for the variations of the vertical observation angle.
Another object of the present invention is to provide a half tone liquid crystal display circuit which produces a multi-color display of high quality.
The aforementioned and other objects and novel features of the present invention will become apparent from the following description to be made with reference to the accompanying drawings.